The present invention relates to a method for manufacturing an ethylene-vinyl acetate copolymer (EVA) and an apparatus for the same.
Saponified ethylene-vinyl acetate copolymers (ethylen-vinyl alcohol copolymer: EVOH) have excellent melt moldability, gas barrier properties, water resistance, oil resistance, antistatic properties and mechanical strength, and are used as various types of packages in the form of films, sheets and containers etc. Increasing demand for EVOH has emphasized the importance of the technology for manufacturing EVOH and its precursor (EVA), especially for the continuous manufacture of EVA.
JP 60(1985)-53513 A discloses a method for the continuous manufacture of EVA over a long period. To suppress overheating of the polymerization vessel and adhesion of scale to the vessel, the method includes contacting an ethylene containing gas with vinyl acetate or a vinyl acetate solution in a heat exchanger in countercurrent to each other. In this method, as the amount of raw materials increases to boost the EVA output, the gas flow in the heat exchanger inhibits the smooth passage of the vinyl acetate (solution). Due to this phenomenon, which is called a flooding phenomenon, the output of EVA is limited by the capacity of the heat exchanger.
Viewed from one aspect, the present invention thus provides a method for the manufacture of an ethylene-vinyl acetate copolymer (EVA) in which ethylene and vinyl acetate are copolymerized in a polymerization solution which comprises ethylene, vinyl acetate, a polymerization solvent and a polymerization initiator. This method comprises:
(a) introducing an ethylene containing gas and a vinyl acetate containing liquid into a heat exchanger, said gas comprising ethylene vaporized from the polymerization solution;
(b) bringing said gas into contact with said liquid in the heat exchanger so as to dissolve at least a portion of the ethylene in said gas into said liquid; and
(c) introducing said liquid from the heat exchanger into the polymerization solution.
In this method, the ethylene containing gas and the vinyl acetate containing liquid flow in parallel with each other in the heat exchanger.
The vinyl acetate containing liquid may be vinyl acetate or a vinyl acetate containing solution. The solution can include the polymerization solvent and preferably is a solution of vinyl acetate in the polymerization solvent.
It is preferable that the heat exchanger has a plurality of tubes and that the vinyl acetate containing liquid flows along the inner surfaces of the tubes and a refrigerant flows along the outer surfaces of the tubes. It is preferable that a refrigerant and the vinyl acetate containing liquid flow in parallel with each other in the heat exchanger.
Temperature T1 (xc2x0 C.) being the temperature of the vinyl acetate containing liquid that is introduced from the heat exchanger into the polymerization solution, and temperature T2 (xc2x0 C.) being the temperature of the polymerization solution, should preferably satisfy the relationship xe2x88x9220xe2x89xa6T1 less than T2.
The EVA preferably contains 5 mol % to 60 mol % of ethylene. An aliphatic alcohol having not more than four carbon atoms is suitable for the polymerization solvent.
Viewed from another aspect, the present invention provides a method for the continuous manufacture of EVA containing 5 mol % to 60 mol % of ethylene. This method comprises:
(axe2x80x2) continuously introducing ethylene, a vinyl acetate containing liquid, a polymerization solvent and a polymerization initiator into a polymerization vessel, whereby to form a polymerization solution, wherein at least a portion of said liquid is introduced via a heat exchanger, wherein the vinyl acetate containing liquid is vinyl acetate or a vinyl acetate solution;
(bxe2x80x2) continuously introducing an ethylene containing gas comprising ethylene vaporized from the polymerization solution into the heat exchanger;
(cxe2x80x2) continuously bringing said gas into contact with said liquid in the heat exchanger so as to dissolve at least a portion of the ethylene in said gas into said liquid, wherein said gas and said liquid flow in parallel with each other;
(dxe2x80x2) continuously introducing said liquid from the heat exchanger into the polymerization solution;
(exe2x80x2) continuously vaporizing a portion of the ethylene in the polymerization solution so as to absorb at least a portion of the heat of polymerization in the polymerization solution, wherein said gas in step (bxe2x80x2) includes at least a portion of the ethylene vaporized in step (exe2x80x2); and
(fxe2x80x2) continuously draining a portion of the polymerization solution from the vessel.
Viewed from a further aspect, the present invention provides a method for manufacturing an ethylene-vinyl alcohol copolymer (EVOH) that includes saponifying the EVA obtained by the method herein described. The present invention also provides EVA obtainable by the above method.
The present invention provides an apparatus for manufacturing EVA. The apparatus includes:
a vessel for containing a polymerization solution that includes ethylene, vinyl acetate, a polymerization solvent and a polymerization initiator;
a heat exchanger;
a first pipe for introducing a vinyl acetate containing liquid into the heat exchanger, said first pipe having one end that is connected to the heat exchanger;
a second pipe for introducing an ethylene containing gas from the vessel into the heat exchanger, said second pipe having one end that is connected to the heat exchanger; and
a third pipe for introducing said liquid from the heat exchanger into the vessel.
In this apparatus, said one end of the first pipe and said one end of the second pipe are connected to a portion of the heat exchanger such that in use said gas and said liquid flow in parallel with each other in said heat exchanger.
Said portion of the heat exchanger is preferably connected to an upper portion of the heat exchanger and the third pipe should have one end that is connected to a lower portion of the heat exchanger (i.e. a portion lower than the portion to which the first and the second pipes are connected).
It is preferable that the heat exchanger is a wetted-wall multi-tubular heat exchanger. The apparatus can further include a fourth pipe for draining the polymerization solution including EVA from the vessel.